One type of optical gyroscope, described in U.S. Pat. No. 4,280,766 by Willis C. Goss et al. transmits a laser light pulse, splits the pulse into two components, and passes the two components in opposite directions through a coil of optical fiber of a considerable length such as 10 kilometers. The two light pulse components that exit from the coil are directed onto a photodetector to detect the difference in their phase. If the coil is turning, the light component moving in the direction of rotation will arrive at the detector later than the light component moving opposite to the direction of rotation, and therefore the two components will be out of phase. The greater the phase difference (up to 180.degree.) the lower the amount of light shining on the detector. In order to detect rotation with high sensitivity and accuracy, the amount of light incident on the detector must be detected with great accuracy.
A major difficulty which arises in the accurate detection of light falling on the photodetector is the large amounts of light that are received which do not represent the returned light components. A major source of such "noise" arises as reflections from optical components lying between the laser and the coil of optical fiber, such as at the surfaces of beam splitters and other optical components. These reflections are received only a very short time after the transmission of the laser pulse. Another source of noise is backscatter from the optic fiber, which decreases exponentially until it is at a very low level immediately before the detection of the pulse components.
Several types of devices can be used to detect the light components, but not the reflections and backscatter light, by turning on the detector just before the light components are expected to be received. One type of detector is a silicon or germanium photodetector which can be switched on just before the returned components are expected. However, the switching on of a silicon or germanium photodetector results in the creation of electrical noise which adds an unpredicable amount to the electrical charge generated by the detection of light from the interfering components. An image intensifier tube could be gated on just prior to the expected time of return of the light pulse components, and its output delivered to a silicon or germanium photodetector to increase the signal to noise ratio. However, the gain of available image intensifiers is not highly stable, and typically only 15% of the photons reaching such an intensifier tube result in the generation of an electron so the tubes are fairly noisy. Also, such tubes and their power supplies are typically bulky and expensive.